1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a thin film transistor array substrate and a fabricating method thereof.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls light transmittance of liquid crystal using an electric field, to thereby display an image. Liquid crystal displays are largely classified into two types: a vertical electric field type and a horizontal electric field type. This classification is based upon the direction that the electric field drives the liquid crystal.
The liquid crystal display of vertical electric field type drives a liquid crystal in a twisted nematic (TN) mode with a vertical electric field formed between a pixel electrode and a common electrode arranged in opposition to each other on the upper and lower substrates. The liquid crystal display of vertical electric field applying type has an advantage of a high aperture ratio while having a drawback of a narrow viewing angle about 90 degrees. The liquid crystal display of horizontal electric field type drives a liquid crystal in an in plane switch (IPS) mode with a horizontal electric field between the pixel electrode and the common electrode arranged in parallel to each other on the lower substrate. The liquid crystal display of horizontal electric field type has an advantage of a wide viewing angle at about 160 degrees. Hereinafter, the liquid crystal display of horizontal electric field applying type will now be described in detail.
FIG. 1 is a plan view showing a structure of a thin film transistor array substrate of the related art horizontal electric field type LCD, and FIG. 2 is a cross-sectional view of the thin film transistor array substrate taken along the line II-II′ in FIG. 1. Referring to FIG. 1 and FIG. 2, the thin film transistor array substrate includes gate lines 2 and data lines 4 provided on a lower substrate 1 to cross each other. A thin film transistor 30 is provided at each crossing. A pixel electrode 22 and a common electrode 24 are provided in a pixel area 5 defined between the gate lines 2 and the data lines 4. The pixel electrode 22 and the common electrode 24 are to form a horizontal electric field. A connection line 26 connects the common electrode 24 to common electrodes of adjacent pixel areas.
One of the gate lines 2 applies a gate signal to a gate electrode of the thin film transistor 30. One of the data lines 4 applies a pixel signal to the pixel electrode 22 via a drain electrode 10 of the thin film transistor 30. The common line 26 is formed in parallel with the gate line for a pixel area 5 and is connected to the common electrodes 24 to apply a reference voltage for driving the liquid crystal 24.
The thin film transistor 30 allows the pixel signal of the data line 4 to be charged and maintained on the pixel electrode 14 in response to the gate signal of the gate line 2. The thin film transistor 30 includes a gate electrode 6 connected to one of the gate lines 2, a source electrode 8 connected to one of the data lines 4, and a drain electrode 10 connected to the pixel electrode 22. Further, the thin film transistor 30 includes an active layer 14, which is on a gate insulating film 12, overlapping the gate electrode 6, a channel between a source electrode 8 and a drain electrode 10, and an ohmic contact layer 16 for making ohmic contacts with the source electrode 8 and the drain electrode 10.
The pixel electrode 22 is provided in the pixel area 5 and is connected to the drain electrode 10 of the thin film transistor 30 via a contact hole 20 through a protective film 18. The pixel electrode 22 includes a first horizontal part 22a connected to the drain electrode 10 and provided in parallel to adjacent gate lines 2 and a second horizontal part 22b overlapping the common line 26. Further, the pixel electrode includes a finger parts 22c provided in parallel to the common electrode 24 between the first and second horizontal parts 22a and 22b. 
The common electrodes 24 are connected to the common line 26 and are provided in the pixel area 5. Specifically, the common electrodes 24 is provided in parallel with the finger part 22c of the pixel electrode 22 in the pixel area 5. Accordingly, a horizontal electric field is formed between the pixel electrode 22 to which a pixel signal is supplied via the thin film transistor 30 and the common electrode 24 to which a reference voltage is supplied via the common line 26.
The horizontal electric field is formed between the finger parts 22c of the pixel electrode 22 and the common electrodes 24. Liquid crystal molecules arranged in the horizontal direction between the thin film transistor array substrate and the color filter array substrate by such a horizontal electric field are rotated due to a dielectric anisotropy of the liquid crystal molecules. Transmittance of a light transmitting through the pixel area 5 is differentiated depending upon the extent of rotation by the liquid crystal molecules, thereby implementing a gray level scale.
In the related art liquid crystal display panel of the horizontal electric field type, the liquid crystal positioned between the common electrode and the pixel electrode 22 is arranged in the same direction as the rubbing direction when no electric field is applied and at an angle when the electric field is applied. The liquid crystal has a maximum transmittance when it is aligned at an angle of 45 degrees. To this end, a horizontal electric field between the common electrode 24 and the pixel electrode 22 must maintain a specific voltage, for example, 6V as shown in FIG. 3.
The liquid crystal has a problem in that, when the horizontal electric field between the common electrode 24 and the pixel electrode 22 is more than the specific voltage, it tends to move further toward the horizontal direction of the electric field, which is more than 45 degrees, such that a deterioration in transmittance occurs.